Thicken Walls of Building without Distortion
Solidify &amp; Shrink/Fatten

A way to give thickness to planar object is the Solidify modifier you can find in the modifier tab of the object. As you want to keep the profile even, you should enable the Even Thickness checkbox:



This will keep the wall straight and clean till your current topology allows.

Once the modifier has been apply, the correct way to increase the wall's thickness only on the seleced faces is to use the Shrink/Fatten tool you can find in the Transform tab of the Tooshelf or call it with Alt+S shortcut.



Press Alt (or S) while ruiing the command to enable Even Thickness option to keep walls straight during the transformation.